Sneezing is an involuntary action because it's something we cannot control.
B) Flu is easily transmitted from person to person.
If there is a very big population density, it means more people are present in a small amount of space. Thus they will come into contact more often and the virus is likelier to spread more rapidly.
Plant cells have what is perhaps the most complex outer coverings. Plant cell walls are made largely of cellulose which forms strong, highly rigid, almost indigestible coverings that protect the cell and gives it shape.
